An electric resistance welded steel pipe is produced by subjecting a steel strip to continuous cold forming into a substantially circular cross-section (a pipe-shaped body) using a plurality of rolls, butting the opposed end faces of the pipe-shaped body against each other, and subjecting the butted parts (butted portions) to welding (electric resistance welding) by applying a high-frequency current to the butted portions under application of pressure to thereby obtain a pipe body having a seam (the electric resistance welded steel pipe). During the electric resistance welding, the butted portions are heated to their melting point or higher by resistance heating under application of pressure, so that the steel strip itself serves as a joint metal that joins the butted end faces together. Therefore, the electric resistance welding is essentially classified as fusion welding.
The performance of resistance welded steel pipes has been improved dramatically by a high-frequency welding technique that has advanced significantly since the 1970s, a heat input controlling-monitoring technique that has been developed and used since the 1980s, an online heat treatment technique for seams, and other techniques. Therefore, the electric resistance welded steel pipes are being widely used for applications such as oil country tubular goods and line pipes having an outer diameter of 26 inches or less and a wall thickness of 1 inch or less and used for mining and transportation of petroleum, gas, etc.
However, from the viewpoint of the reliability of the electric resistance weld zones of electric resistance welded steel pipes, the electric resistance welded steel pipes are used only for applications with no severe specification requirements for the electric resistance weld zones. Therefore, various proposals have recently been made in order to improve the reliability of the electric resistance weld zones.
For example, Patent Literature 1 describes an electric resistance welded steel pipe with high cracking resistance and excellent sour resistance in an environment containing wet hydrogen sulfide. In the technique described in Patent Literature 1, the electric resistance welded steel pipe is made of Al-deoxidized steel containing 0.0012% by mass or more of Ca. In this steel pipe, the ratio of Ca/Al in the steel is 0.10% or less, and oxide-based inclusions are contained in an area extending 100 μm in opposite directions from the electric resistance-butt weld surface. In a cross section perpendicular to the butt weld surface and also perpendicular to the axial direction of the pipe, these oxide-based inclusions include inclusions having a shape that is elongated in the direction of the sheet thickness such that the ratio of the length in the thickness direction to the length in the circumferential direction of the pipe is 2 or more and the major axis is 10 μm or more. In the area extending 100 μm in opposite directions from the butt weld surface, the density of the elongated inclusions in the cross section is 5 per mm2 or less. It is stated that this can prevent the occurrence of hydrogen-induced blistering even in a severe environment.
Patent Literature 2 describes a gas shielded welding method for an electric resistance welded steel pipe. In the technique described in Patent Literature 2, the inner side of the pipe is washed with mist to remove suspended scale after fin pass forming but before welding. When local sealing is performed on the weld zone, a sealing device on the inner side of the pipe, except for its holding rollers, is not in contact with the pipe. It is stated that this can prevent the scale from remaining in the weld zone, so that the toughness of the weld zone is significantly improved.
Patent Literature 3 describes a high-strength electric resistance welded line pipe. The electric resistance welded line pipe described in Patent Literature 3 has a composition containing, in mass %, C: more than 0.04 to 0.08%, Si: 0.1 to 0.3%, Mn: more than 1.6 to 2.0%, P: 0.02% or less, S: 0.003% or less, Nb: 0.04 to 0.08%, V: 0.05 to 0.1%, Ni: 0.1 to 0.5%, Cu: 0.1 to 0.5%, Mo: 0.05 to 0.20%, Ti: 0.01 to 0.03%, Al: 0.05% or less, and N: 0.005% or less such that Ni, Cu, and Mo satisfy a specific relation. The metallographic structure of the electric resistance welded line pipe is an acicular ferrite structure with an average crystal grain size of 5 μm or less. The tensile strength of the line pipe in its circumferential direction after flattening is 700 N/mm2 or more, and the 0.5% proof stress of the line pipe is 550 N/mm2 or more. The area occupied by oxides in the electric resistance-butt weld zone is 0.1% (corresponding to 1,000 ppm) or less. The electric resistance welded line pipe described in Patent Literature 3 is an electric resistance welded steel pipe having an outer diameter of 200 to 610 mm and a wall thickness/outer diameter ratio (t/D) of 2% or less and manufactured using a hot rolled steel coil through steps including cold roll forming, electric resistance welding, seam heat treatment, and sizing. It is stated that the electric resistance weld zone has soundness comparable to that of the base material, so that the line pipe can be further reduced in thickness.
Patent Literature 4 describes an electric resistance welded boiler steel pipe. The electric resistance welded boiler steel pipe described in. Patent Literature 4 contains, in mass %, C: 0.01 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, and Cr: 0.5 to 3.5%. P is limited to 0.030% or less, S is limited to 0.010% or less, and 0 is limited to 0.020% or less. (Si %)/(Mn %+Cr %) is from 0.005 to 1.5 inclusive. The area fraction of a ternary mixed oxide of SiO2, MnO, and Cr2O3 generated during electric resistance welding is 0.1% (corresponding to 1,000 ppm) or less. The number of defects in the electric resistance weld zone is small, and the electric resistance welded boiler steel pipe is excellent in creep rupture strength and toughness.
To obtain an electric resistance welded steel pipe with the electric resistance weld zone having stable performance, it is necessary to appropriately maintain the conditions of electric resistance welding to thereby obtain stable weld quality (seam quality). Therefore, it is important to stabilize the shape of the edges of a steel strip used as a steel pipe raw material, to adjust the positions of the edges during forming and welding, and to stabilize the amount of heat input during electric resistance welding.
When electric resistance welding is performed under the conditions in which optimal welding heat input is obtained, the butted portions of a pipe-shaped body (the edges of a steel strip) are melted sufficiently to form droplets and thereby pressure-welded together. When the amount of heat input is small, the butted portions are pressure-welded together while droplets are not formed sufficiently. Therefore, in a weld zone welded under the conditions in which the amount of heat input is small, a large amount of oxide is observed in the weld surface when the weld zone is ruptured along the weld surface, as shown in FIG. 1(a) in Non Patent Literature 1. A weld zone in which a large amount of oxide is observed in its fractured surfaces is generally called a cold weld or a cold joint. The weld zone shown in Non Patent Literature 1 is a high-frequency weld zone. This means that, even with high-frequency welding with small variations in the amount of heat input, cold welds (weld defects) can be formed depending on the conditions of welding.